In general, computing devices with graphical user interfaces (GUIs), such as computers and mobile devices, are designed to receive user input using input devices such as a mouse, keyboard, or a number pad. Some computing devices (particularly mobile devices) also include a touch-sensitive screen. Until recently, touch-sensitive screens were generally “single-touch” screens, which process a single touch input at a time. Thus, computing devices with these screens were generally configured to use the touch-sensitive screen to provide a pointer input as an alternative to a mouse. More recently, “multi-touch” touch-sensitive screens have been produced that are capable of receiving multiple touch input simultaneously. For example, iPhones sold by Apple, Inc. of Cupertino, Calif., enable users to use a thumb and forefinger in combination to control the level of zoom in various mobile applications. However, even modern computing devices do not make extensive use of the multi-touch capabilities of these new devices. Thus, it would be useful to have new means for controlling computing devices based on multi-touch user input.